The present invention is generally directed to spot cooling devices for use in conjunction with integrated circuit electronic devices principally, though not exclusively, in the form of Single Chip Modules (SCMs). More particularly, the present invention is directed to a method and apparatus for improving air flow through the fins of a heat sink. Even more particularly, the present invention is directed to a method and apparatus for enhancing the flow of air in spot cooling devices which comprise combinations of fans and heat sinks.
As a result of the quest for faster and more functional electronic processing chips, single chip module heat dissipation levels are climbing beyond that which can be cooled using conventional heat sinks. In many applications, spot cooling methods are employed for high powered single chip modules. These single chip modules include not only processor chips but also communications and other chips which are operated at high power levels due to chip circuit density, speed of operation or both. Spot coolers are employed because the acceptable air speeds within various electronic systems are not sufficient to provide proper cooling. Additionally, spot coolers are employed because available space for cooling hardware is limited. And lastly, spot coolers are employed because the electronic system may not have been originally designed to accommodate a high powered component which is currently being employed or which is desired to be employed.
A popular, but suboptimal, solution to this thermal challenge involves attaching a miniature tubeaxial fan on to the top of a low profile heat sink. One manufacturing technique to achieve this end is the placement of the tubeaxial fan in a pocket in an extruded aluminum, parallel-plate heat sink. Such pockets are specifically designed for the insertion of these fans. Another manufacturing technique uses die casting to form parallel plate heat sinks with a cast shape which includes a pocket to receive a small fan.
The fans, or more generally, the air moving devices, with which this invention is most directly concerned, are of the so-called muffin fan variety. The relevant features of these fans or air moving devices with respect to the present invention is that the fans include a centrally disposed motor together with a set of fan blades which are rotated by the motor so as to cause the fan blades to rotate within an annular volume. The fan motor itself may form the central region bounded by this annular volume but generally, the motor itself is not required to be present in a region which is actually surrounded by the annular volume. For purposes of the present invention, the most relevant feature of the air moving device which is employed is that the device causes air to flow in an annular volume toward the object which is desired to be cooled.
This approach suffers from a very poor match between the heat distribution pattern within the electronic chip device and the air flow velocity profile of the tubeaxial fan. Specifically, the heat dissipated by the heat sink is typically concentrated as a high thermal flux in the center of the electronic package, whereas the fan delivers little or no air flow to the central region of the heat sink. Rather, the fan provides the outer portion of the heat sink with the highest air flow velocity and heat transfer coefficient. This means that the heat path is from the chip to the heat sink base, laterally through the base to the outer fins and finally into the air flow. However, lateral heat conduction through the heat sink base smears the flux concentration away from the center at the cost of conduction (spreading) losses. This heat flow pattern ultimately results in higher electronic device junction temperatures. Consequently, the present inventors have perceived that a design which delivers air flow to the fins in the center of the heat sink provides superior cooling, lower junction temperature and therefore greater circuit reliability.